Adaptive Signal Reception

ABSTRACT

A robotic lawnmower system, comprising a robotic lawnmower and a charging station, said charging station comprising a signal generator being configured to transmit a boundary signal (A) utilizing CDMA coding having a frame length corresponding to a full frame length through a boundary wire, wherein said signal generator is configured to transmit said boundary signal in subframes, and wherein said robotic lawnmower is configured to receive said boundary signal by detecting magnetic fields generated by the boundary signal, and wherein the robotic lawnmower is configured to determine a set of conditions; and to determine if said set of conditions correspond to a first set of conditions, and if so listen to the subframes, and to determine if said set of conditions correspond to a second set of conditions, and if so listen to the full frames.

TECHNICAL FIELD

This application relates to automatic lawnmowers and in particular to amethod for performing improved reception of a transmitted signal on thereceiver side.

BACKGROUND

Automated or robotic power tools such as robotic lawnmowers are becomingincreasingly more popular. In a typical deployment, a work area, such asa garden, is enclosed by a boundary cable with the purpose of keepingthe robotic lawnmower inside the work area. The robotic lawnmower istypically also configured to communicate with a charging station locatedin the work area and connected to the boundary cable.

Using standardised coding such as Gold codes, has the obvious advantagethat new coding schemes need not be invented. However, the inventorshave realized that the frame length commonly used for CDMA coding, suchas Gold coding, when used with technology commonly used for lawnmowersystems leads to a transmission time for the entire frame that is in theorder of seconds, such as 1 second, 0.5 seconds or up to 0.5 seconds.Such time spans may be unpractical in real life implementations as arobotic lawnmower operating using such time frames would move a distancethat could not be neglected before being able to decode the entireframe. The robotic lawnmower may thus be rendered unable to detectwhether it is still within the work area or not.

This would for practical reasons render gold coding inoperable forrobotic lawnmower systems. To overcome this, the inventors realized thatby dividing a frame as per above, shorter segments of the entire frame,i.e. sub frames, may be used to control the robotic lawnmower.

SUMMARY

The inventors have realized that by configuring the signal generator totransmit sub frames and by configuring the robotic lawnmower to operateaccording to sub frames, the gold coding of CDMA systems, may be usedalong with contemporary hardware technologies, commonly used in roboticlawnmower systems, such as transmitting a signal through a boundarycable, which signals is picked up by coil-based sensors in the roboticlawnmower.

It is an object of the teachings of this application to overcome theproblems listed above by providing a robotic lawnmower system,comprising a robotic lawnmower and a charging station, said chargingstation comprising a signal generator being configured to transmit aboundary signal (A) utilizing CDMA coding having a frame lengthcorresponding to a full frame length through a boundary wire, whereinsaid signal generator is configured to transmit said boundary signal insubframes, and wherein said robotic lawnmower is configured to receivesaid boundary signal by detecting magnetic fields generated by theboundary signal, and wherein the robotic lawnmower is configured todetermine a set of conditions; and to determine if said set ofconditions correspond to a first set of conditions, and if so listen tothe subframes, and to determine if said set of conditions correspond toa second set of conditions, and if so listen to the full frames.

It is also an object of the teachings of this application to overcomethe problems listed above by providing a method for controlling arobotic lawnmower system, comprising a robotic lawnmower and a chargingstation, said charging station comprising a signal generator beingconfigured to transmit a boundary signal (A) utilizing CDMA codinghaving a frame length corresponding to a full frame length through aboundary wire, wherein said signal generator is configured to transmitsaid boundary signal in subframes, and wherein said robotic lawnmower isconfigured to receive said boundary signal by detecting magnetic fieldsgenerated by the boundary signal, and wherein the method comprisesdetermining a set of conditions; and determining if said set ofconditions correspond to a first set of conditions, and if so listen tothe subframes, and determining if said set of conditions correspond to asecond set of conditions, and if so listen to the full frames.

Other features and advantages of the disclosed embodiments will appearfrom the following detailed disclosure, from the attached dependentclaims as well as from the drawings. Generally, all terms used in theclaims are to be interpreted according to their ordinary meaning in thetechnical field, unless explicitly defined otherwise herein. Allreferences to “a/an/the [element, device, component, means, step, etc]”are to be interpreted openly as referring to at least one instance ofthe element, device, component, means, step, etc., unless explicitlystated otherwise. The steps of any method disclosed herein do not haveto be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail under reference to theaccompanying drawings in which:

FIG. 1A shows an example of a robotic lawnmower according to oneembodiment of the teachings herein;

FIG. 1B shows a schematic view of the components of an example of arobotic lawnmower according to one embodiment of the teachings herein;

FIG. 2 shows an example of a robotic lawnmower system according to theteachings herein;

FIG. 3 shows a schematic view of a protocol according to one embodimentof the teachings herein;

FIG. 4 shows a schematic flowchart for a general method according to theteachings herein;

FIGS. 5A, 5B and 5C are schematic views of a robotic lawnmower systemaccording to the teachings herein;

FIG. 6 shows a schematic flowchart for a general method according to theteachings herein;

FIG. 7 shows a schematic flowchart for a general method according to theteachings herein;

FIG. 8 shows a amplitude frequency diagram for a transmitted signalaccording to one embodiment of the teachings herein;

FIG. 9 shows a amplitude frequency diagram for a received signalaccording to one embodiment of the teachings herein;

FIG. 10 shows a schematic flowchart for a general method according tothe teachings herein; and

FIG. 11 shows a schematic flowchart for a general method according tothe teachings herein.

DETAILED DESCRIPTION

The disclosed embodiments will now be described more fully hereinafterwith reference to the accompanying drawings, in which certainembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

It should be noted that all indications of rotational speeds, timedurations, work loads, battery levels, operational levels etc. are givenas examples and may be varied in many different ways as would beapparent to a skilled person. The variations may be for individualentities as well as for groups of entities and may be absolute orrelative.

FIG. 1A shows a perspective view of a robotic working tool 100, hereexemplified by a robotic lawnmower 100, having a body 140 and aplurality of wheels 130 (only one shown). As can be seen, the roboticlawnmower 100 may comprise charging skids 132 for contacting contactplates (not shown in FIG. 1, but referenced 230 in FIG. 2) when dockinginto a charging station (not shown in FIG. 1, but referenced 210 in FIG.2) for receiving a charging current through, and possibly also fortransferring information by means of electrical communication betweenthe charging station and the robotic lawnmower 100.

FIG. 1B shows a schematic overview of the robotic working tool 100, alsoexemplified here by a robotic lawnmower 100, having a body 140 and aplurality of wheels 130.

It should be noted that even though the description given herein will befocused on robotic lawnmowers, the teachings herein may also be appliedto robotic cleaners such as robotic vacuum cleaners and/or robotic floorcleaners, robotic ball collectors, robotic mine sweepers, roboticfarming equipment, or other robotic work tools to be employed in a workarea defined by a boundary cable.

In the exemplary embodiment of FIG. 1B the robotic lawnmower 100 has 4wheels 130, two front wheels 130′ and the rear wheels 130″. At leastsome of the wheels 130 are drivably connected to at least one electricmotor 150. It should be noted that even if the description herein isfocused on electric motors, combustion engines may alternatively be usedpossibly in combination with an electric motor.

In the example of FIG. 1B, each of the rear wheels 130″ is connected toa respective electric motor 150. This allows for driving the rear wheels130″ independently of one another which, for example, enables steepturning.

The robotic lawnmower 100 also comprises a controller 110. Thecontroller 110 may be implemented using instructions that enablehardware functionality, for example, by using executable computerprogram instructions in a general-purpose or special-purpose processorthat may be stored on a computer readable storage medium (disk, memoryetc) 120 to be executed by such a processor. The controller 110 isconfigured to read instructions from the memory 120 and execute theseinstructions to control the operation of the robotic lawnmower 100including, but not being limited to, the propulsion of the roboticlawnmower. The controller 110 may be implemented using any suitable,publically available processor or Programmable Logic Circuit (PLC). Thememory 120 may be implemented using any commonly known technology forcomputer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR,SDRAM or some other memory technology.

The robotic lawnmower 100 may further have at least one sensor 170; inthe example of FIG. 1 there are four sensors divided into a first sensorpair 170′ arranged at a front of the robotic lawnmower 100 and a secondsensor pair 170″, respectively arranged at the rear of the roboticlawnmower 100 to detect a magnetic field (not shown) and for detecting aboundary cable and/or for receiving (and possibly also sending)information from a signal generator (will be discussed with reference toFIG. 2). The sensors 170 may thus be arranged as front sensors 170′ andrear sensors 170″.

In some embodiments, the sensors 170 may be connected to the controller110, and the controller 110 may be configured to process and evaluateany signals received from the sensor pairs 170, 170′. The sensor signalsmay be caused by the magnetic field being generated by a control signalbeing transmitted through a boundary cable. This enables the controller110 to determine whether the robotic lawnmower 100 is close to orcrossing a boundary cable, or inside or outside an area enclosed by theboundary cable. This also enables the robotic lawnmower 100 to receive(and possibly send) information from the control signal.

The robotic lawnmower 100 also comprises a grass cutting device 160,such as a rotating blade 160 driven by a cutter motor 165. The grasscutting device being an example of a work tool 160 for a robotic worktool 100. The cutter motor 165 is connected to the controller 110 whichenables the controller 110 to control the operation of the cutter motor165. The controller may also be configured to determine the load exertedon the rotating blade, by for example measure the power delivered to thecutter motor 165 or by measuring the axle torque exerted by the rotatingblade. The robotic lawnmower 100 also has (at least) one battery 180 forproviding power to the motors 150 and the cutter motor 165.

The robotic lawnmower 100 may further comprise at least one supplementalnavigation sensor 190, such as a deduced reckoning navigation sensor forproviding signals for deduced reckoning navigation, also referred to asdead reckoning. Examples of such deduced reckoning navigation sensor(s)190 are odometers and compasses. The supplemental navigation sensor mayalso or alternatively be implemented as a vision navigation system, orUltra Wide Band radio navigation system to mention a few examples. Thesupplemental sensor 195 will hereafter be exemplified through thededuced reckoning sensor.

The robotic lawnmower 100 may further be arranged with a wirelesscommunication interface 197 for communicating with other devices, suchas a server, a personal computer or smartphone, or the charging station.Examples of such wireless communication devices are Bluetooth™, GlobalSystem Mobile (GSM) and LTE (Long Term Evolution), to name a few.

In addition, the robotic lawnmower 100 may be arranged with collisionsensor means for detecting when the robotic lawnmower 100 runs into anobstacle. The collision sensor means may be one or more separate sensors(such as accelerometers, pressure sensors or proximity sensors) arrangedin or on the housing of the robotic lawnmower 100 and capable ofdetecting an impact caused by a collision between the robotic lawnmower100 and an obstacle. Alternatively, the collision sensor means may beimplemented as a program routine run by the controller 110, beingeffective to detect a sudden decrease of the rotational speed of any ofthe drive wheels 130″ and/or sudden increase in the drive current to theelectric motor 150.

FIG. 2 shows a schematic view of a robotic working tool system 200 inone embodiment. The schematic view is not to scale. The robotic workingtool system 200 comprises a charging station 210 and a boundary cable250 arranged to enclose a work area 205, in which the robotic lawnmower100 is supposed to serve. Adjacent to the work area 205 is another workarea 205′ enclosed by a boundary 250′. Although not shown, in someembodiments, the other work area 205′ may also comprise a chargingstation and robotic working tool deployed within.

As with FIG. 1, the robotic working tool is exemplified by a roboticlawnmower, but the teachings herein may also be applied to other roboticworking tools adapted to operate within a work area defined by aboundary cable. The work area is defined by a signal being transmittedthrough the boundary cable, which gives rise to a magnetic field thatthe sensor s 170 of the robotic lawnmower detects and based on this, therobotic lawnmower may determine whether it is inside or outside the workarea and also when it crosses the boundary cable.

The charging station may have a base plate for enabling the roboticlawnmower to enter the charging station in a clean environment and forproviding stability to the charging station 210.

The charging station 210 has a charger 220, in this embodiment coupledto two charging plates 230. The charging plates 230 are arranged toco-operate with corresponding charging plates (not shown) of the roboticlawnmower 100 for charging a battery 180 of the robotic lawnmower 100.

The charging station 210 also has, or may be coupled to, a signalgenerator 240 for providing a control signal 245 to be transmittedthrough the boundary cable 250. The signal generator thus comprises acontroller for generating the control signal. The control signal 245comprises an alternating current, such as a continuously or regularlyrepeated current signal. The control signal is in one embodiment a CDMAsignal (CDMA—Code Division Multiple Access). As is known in the art, thecurrent signal will generate a magnetic field around the boundary cable250 which the sensors 170 of the robotic lawnmower 100 will detect. Asthe robotic lawnmower 100 (or more accurately, the sensor 170) crossesthe boundary cable 250 the direction of the magnetic field will change.The robotic lawnmower 100 will thus be able to determine that theboundary cable has been crossed, and take appropriate action bycontrolling the driving of the rear wheels 130″ to cause the roboticlawnmower 100 to turn a certain angular amount and return into the workarea 205. For its operation within the work area 205, in the embodimentof FIG. 2, the robotic lawnmower 100 may use the satellite navigationdevice 190, supported by the deduced reckoning navigation sensor 195 tonavigate the work area 205.

The use of more than one sensor 170 enables the controller 110 of therobotic lawnmower 100 to determine how the robotic lawnmower 100 isaligned with relation to the boundary cable 250 by comparing the sensorsignals received from each sensor 170. This enables the roboticlawnmower to follow the boundary cable 250, for example when returningto the charging station 210 for charging. Optionally, the chargingstation 210 may have a guide cable 260 for enabling the roboticlawnmower to find the entrance of the charging station 210. In someembodiments the guide cable 260 is formed by a loop of the boundarycable 250. In some embodiments the guide wire 260 is used to generate amagnetic field for enabling the robotic lawnmower 100 to find thecharging station without following a guide cable 260.

Additionally, the robotic lawnmower 100 may use the satellite navigationdevice 190 to remain within and map the work area 205 by comparing thesuccessive determined positions of the robotic lawnmower 100 against aset of geographical coordinates defining the boundary 250, obstacles,keep-out areas etc of the work area 205. This set of boundary definingpositions may be stored in the memory 120, and/or included in a digital(virtual) map of the work area 205. The boundary 250 of the work area205 may also be marked by a boundary cable supplementing the GNSSnavigation to ensure that the robotic work tool stays within the workarea, even when no satellite signals are received.

The charging station 210 may also be arranged (through the signalgenerator 220) to emit a so-called F-field, referenced F in FIG. 2. TheF-field is a magnetic field generated around the charging station whichenables a robotic lawnmower to navigate towards the charging station 210without having to follow a guide or boundary cable, simply by navigatingtowards an increased field strength of the F-field.

The charging station 210 may also be arranged (through the signalgenerator 220) to emit a so-called N-field, referenced N in FIG. 2. TheN-field is a magnetic field generated in the base plate 215 of thecharging station which enables a robotic lawnmower to navigate correctlyin the charging station for making contact with the charging plates 230.

The control signal 245 may also be used to provide information Ito therobotic lawnmower 100 from the charging station 210 through the boundarycable 250 and/or the guide wire 260. The information may be transmittedas a coded message and may relate to an identity for the roboticlawnmower system 200, such as an identity of the robotic lawnmowerand/or an identity for the charging station 210 (or rather the signalgenerator), a command to be executed by the robotic lawnmower 100 and/ordata that the robotic lawnmower may base an operating decision upon,such as weather data.

In one embodiment, the information I carried in the control signal 245is coded using a CDMA (Code Division Multiple Access) coding scheme. Infact, the whole signal A is coded using CDMA, so the information I, andthe portions relating to F, G and N signals are also coded using CDMA:As CDMA allows for more than one transmitter to transmit informationsimultaneously using the same frequency, and as the magnetic wavesgenerated by a current signal in a cable all propagate through the samemedium this allows for having more than one work area 205, 205′ beingserviced by each a robotic lawnmower 100 and its corresponding signalgenerator 240 adjacent one another, while still being able to transmit acoded message that is directed at a specific robotic lawnmower 100.

In some embodiments the robotic lawnmower 100 may also be arranged tosend a signal to the charging station 210 to provide status updates,request data and/or for establishing a two-way communication. Therobotic lawnmower may be arranged with a signal generator (not shown)and an antenna for transmitting electromagnetic signals (not shown). Theelectromagnetic signals may be transmitted directly to the chargingstation 210 or be picked up by the boundary cable 250, the boundarycable then double acting as an antenna. In the following, focus will beon communication from the charging station 210 (via the signal generator240) to the robotic lawnmower 100.

The robotic lawnmower listens or detects the signal by receiving thevoltages generated by the sensor as it detects the magnetic fieldsgenerated by the signal. The received signal is then auto correlatedwith a reference signal. The autocorrelation may be shifted in time,i.e. synchronized, to provide as good a correlation as possible. Thiswill enable the robotic lawnmower to compensate for any differences ordrifts in the clock of the signal generator and the clock of the roboticlawnmower. In one embodiment the CDMA signal 245 is coded using Goldcodes. In the field of robotic lawnmowers a normal frame length for Goldcodes is in the order of 2047 bits, however to speed up the system, therobotic lawnmower system according to the teachings herein may beadapted to use a different frame length. The protocol used in oneembodiment consists of two Gold frame lengths of each 2047 bitstotalling 2×2047 bits, which frame is divided into smaller frames, forexample 7 frames of each 584 bits, or 10 frames of each 409 bits. Itshould be noted that other number of frames may also be used.

Every other frame (1, 3, 5, 7 . . . , n) is used to send the controlsignal. Every other frame (2, 4, 6, 8, . . . , n+1) is used to send theF-field, the N-field and the guide signal(s), each being transmitted ina further sub frame. In one embodiment, time division is used fortransmitting the F-field, the N-filed and any information I that may beneeded to be transmitted to (from) the robotic lawnmower 100. Oneexample of such a protocol is shown in FIG. 3 where the upper figureshows the general format, and the lower figure shows an example. In FIG.3 guide signal is abbreviated GS and there are three guide cables, eachtransmitting a guide signal; GS1, GS2, and GS3, and control signal isabbreviated CS. It should be noted that the time scales for the upperfigure and the lower figure are not the same, the upper showing twoframes and the lower showing 8 frames.

It should be noted that the control signal CS is transmitted through theboundary cable 250, the guide signals are transmitted through theirrespective guide cables, the F field is transmitted through itsrespective cable and the N field is transmitted through its respectivecable. The information bits I may be transmitted through any, someand/or all cables depending on the information and the design chosen.

It should also be noted that the number of sub frames my of coursediffer from system to system, depending on the systems capabilities. Forexample, in a robotic working tool system not having an F field, theF-field signal will not be transmitted, and in a robotic working toolsystem not having three guide cables, the number of sub frames used forthe guide signals would also differ.

Using standardised coding such as Gold codes, has the obvious advantagethat new coding schemes need not be invented. However, the inventorshave realized that the frame length commonly used for CDMA coding, suchas Gold coding, when used with technology commonly used for lawnmowersystems leads to a transmission time for the entire frame that is in theorder of seconds, such as 1 second, 0.5 seconds or up to 0.5 seconds.Such time spans may be unpractical in real life implementations as arobotic lawnmower operating using such time frames would move a distancethat could not be neglected before being able to decode the entireframe. The robotic lawnmower may thus be rendered unable to detectwhether it is still within the work area or not.

This would for practical reasons render gold coding inoperable forrobotic lawnmower systems. To overcome this, the inventors realized thatby dividing a frame as per above, shorter segments of the entire frame,i.e. sub frames, may be used to control the robotic lawnmower. Byconfiguring the signal generator to transmit sub frames and byconfiguring the robotic lawnmower to operate according to sub frames,the gold coding of CDMA systems, may be used along with contemporaryhardware technologies, commonly used in robotic lawnmower systems, suchas transmitting a signal through a boundary cable, which signals ispicked up by coil-based sensors in the robotic lawnmower.

By dividing the frame into several subframes, for example 10 or as inthe detailed example given above, the robotic lawnmower is enabled totune in and listen for shorter time spans since the control informationis retransmitted more often, or at higher frequencies, and the roboticlawnmower does not need to receive and decode an entire frame beforebeing able to make a control decisions, such as determining the crossingof a cable or if the robotic lawnmower is inside or outside a workingarea.

However, as a longer frame provides for a more robust system, that isless sensitive to interference, the inventors have realized that theymay make use of the duality offered by utilizing subframes, namely thatthe robotic lawnmower is configured to decode and operate according tosubframes under a first set of conditions, and to decode and operateaccording to complete frames under a second set of conditions, and evento decode and operate according to subframes and whole frames under athird set of conditions.

The first set of conditions includes that the received quality signallevel is high, whereby interference is assumingly low and shorter timeframes may be sufficient and provide enough robustness.

The first set of conditions may alternatively or additionally includethat the received signal power is high, whereby the robotic lawnmower isassumingly close to the boundary cable and also possibly that theinterference is not strong enough to affect the reception, and wherebythe shorter time frames may be needed to provide a fast enough controlof the robotic lawnmower.

The second set of conditions includes that the received quality signallevel is low, whereby interference is assumingly high and full framesare needed to provide sufficient robustness.

The second set of conditions may alternatively or additionally includethat the received signal power is low, whereby the robotic lawnmower isassumingly far away from the boundary cable and also possibly that theinterference is strong enough to affect the reception, and whereby thefull frames may be needed to provide a robust enough control of therobotic lawnmower.

The third set of conditions may include that the signal quality level islow, but the signal strength level is high, indicating that the roboticlawnmower is close to the boundary cable but in a noisy environmentwhereby the robotic lawnmower may be configured to listen to thesubframes to make fast control decisions, and listen to the full framesto confirm the control decisions made based on the subframes.

The inventors have also realized that by changing the order of theframes to be transmitted a more robust reception is provided. The tablebelow shows a schematic view of a frame and how the frame is rearrangedin order to provide the more robust reception. As can be seen, theframes are not transmitted in order. A Frame F comprising 5 subframes F1. . . F5 will then be transmitted in the order F1F4F2F5F3, whereby thesubframes and their corresponding bits (assuming 2000 bits) are givenby:

Subframe Bits 1  1-400 2  801-1200 3 1601-2000 4 401-800 5 1201-1600

In one embodiment, this is utilized for the boundary signal A. The othersignals, (guide and so on) are generally too short to utilize from thebenefits associated herewith, but may of course also be transmitted in asimilar manner.

FIG. 4 shows a schematic flowchart for a general method according to theteachings herein. A signal generator transmits 410 a boundary signalutilizing CDMA coding having a frame length through a boundary cable.The signal generator transmits said boundary signal in subframes. Arobotic lawnmower is configured to receive the boundary signal bydetecting magnetic fields generated by the boundary signal. The roboticlawnmower determines a set of conditions 420; and then determines 430 ifsaid set of conditions correspond to a first set of conditions, and ifso listen to the subframes 435, and determining 440 if said set ofconditions correspond to a second set of conditions, and if so listen tothe full frames 445.

It has been realized by the inventors that to simplify the installationprocess for a robotic lawnmower system, the user or installer may begiven an option to install guide wires or not. Using guide wires has thebenefit that the robotic lawnmower may be able to find its way back tothe charging station more effectively than randomly searching for thecharging station or F-field. Due to constrictions on allowed fieldstrengths, the total field strength of the guide signal, including theF-field, must be kept below certain levels. This is a legal requirementto prevent a system from causing too much interference to itssurroundings.

However, the inventors have realized the simple solution that byallotting the time slots for the guide cable(s) G to the F-field F, thefield strength of the F-field may be increased without increasing thefield strength for the whole guide signal, thereby allowing for astronger F-field while staying within the legal requirements.

A controller of the charging station, the controller possibly being thatof the signal generator 240, may thus be configured to determine whethera guide cable is connected or not and if it is detected that the guidecable is not connected, allot or assign the corresponding or associatedtime slot to the F-field. Alternatively, the associated time slot may beassigned to another guide cable, thereby allowing the robotic lawnmowerto find that guide cable more quickly.

Alternatively, a timeslot associated with a guide cable that has notbeen connected, may be (time) shared by the connected guide cables andthe F-field.

In one embodiment the controller may be configured to detect that afirst guide cable and a second guide cable are not connected and allotthe time slot associated with the first guide cable to the F-field, andallot the time slot associated with the second guide cable to a thirdguide cable.

The controller of the charging station is thus also configured tocommunicate through the information field to the robotic lawnmower sothat the controller of the robotic lawnmower may adapt its sensing ofthe (boundary) signal(s).

In one embodiment a user may provide user input to indicate which guidecables are connected or not and the controller may then determine thetime slot to use for F-field transmission accordingly.

FIGS. 5A, 5B and 5C are schematic views of a robotic lawnmower systemwhere an F-field F and three guide cables G1, G2 and G3 are normallyused, as in FIG. 5A. A schematic view of the boundary signal 245 is alsoshown for each robotic lawnmower system, where in FIG. 5A, the boundarysignal has timeslots for the actual boundary signal A and time slotsassociated with the F-field F, a first time slot associated with a firstguide cable G1, a second timeslot associated with a second guide cableG2 and a third time slot associated with a third guide cable G3.

In the example of FIG. 5A, all three guide cables are used and theassociated time slots are allotted accordingly.

In the example of FIG. 5B, only one guide cable G3 is used. As thecontroller of the charging station detects this, the controller allotsthe time slots associated with the unconnected guide cables G1, G2 areallotted to the F-field, and in this instance also to the third guidecable. As a skilled reader would realize, other allotments would also bepossible within the teachings of this document. Both the F-field and thethird guide cable are thus provided with a higher signal level and maythus be found more easily by the robotic lawnmower and thereby simplifythe installation of the robotic lawnmower system, while enabling therobotic lawnmower to find the charging station easily without randomlysearching for it through most of the working area, all while stayingwithin the legal requirements.

In the example of FIG. 5C, no guide cable is used. As the controller ofthe charging station detects this, the controller allots the time slotsassociated with the unconnected guide cables G1, G2, G3 are allotted tothe F-field, which then receives four time slots, thereby significantlyincreasing its field strength enabling the robotic lawnmower to moreeasily find the F-field thereby simplifying the installation of therobotic lawnmower system, while enabling the robotic lawnmower to stillfind the charging station easily without randomly searching for itthrough most of the working area, all while staying within the legalrequirements. In one embodiment the time slots for F-signals replacingguide wire signals are time synchronized to match a correspondingsequential time slot number.

FIG. 6 shows a flowchart for a general method according to herein wherea controller of a charging station detects 610 whether a guide cable isconnected or not, and if the guide cable is not connected, thecontroller allots an associated time slot to another use 620.

In one embodiment, the other use is to allot the associated time slot tothe F-field F 630.

In one embodiment, the other use is to allot the associated time slot toanother guide cable 640.

In one embodiment, the other use is to allot the associated time slot tobe time shared between the F-field F and at least one other guide cable650.

As a robotic lawnmower 100 picks up a signal, through the sensors 170,the picked up signal is analyzed by the controller 110. It could benoted that the picked up signal differs somewhat to the transmittedsignal as if the sensors are based on coils detecting magnetic fieldchanges, the sensors will only be able to detect changes in thesignal(s), that is, only the derivate of the signal(s) is picked up.This would be understood by a skilled person and in the remainder ofthis description no explicit difference will be made between thetransmitted signal and the picked up signal unless specificallyspecified.

As the controller receives a (picked up) signal, the signal is analyzed,which analysis comprises correlating the (picked up) signal to a stored(or calculated) library or referenced signal to determine if the (pickedup) signal originates from the signal generator 220 of the roboticlawnmower system 200—or from another source. The time slots in theanalyzed signal serve to identify if the (picked up) signal istransmitted through the boundary cable or a guide cable.

To establish a synchronization between the signal generator 220 and thecontroller 110 of the robotic lawnmower 100, the controller isconfigured to convolute the (picked up) signal and correlated to thelibrary signal for the same time, that is S(0)=L(0), where S is thepicked up signal and L is the library signal. The (picked up) signal mayalso be correlated to previous or subsequent times t=+/−1, +/−2, etc,i.e. the signal is shifted in time.

As has been discussed above, the robotic lawnmower 100 may comprise atleast one front sensor 170′ and at least one rear sensor 170″. Duringoperation, the robotic lawnmower 100 will distance itself from theboundary cable 250 i.e. to operate within the inner portions of the workarea 254. At such distances, the received signal may be received at alower amplitude or signal strength. The reception will then be moresusceptible to noise and interference. One of the major interferencesources are actually the electric motors and the control signals forthese. Traditionally, the robotic lawnmower is then configured tosynchronize to the received signal using the front sensor(s) 170′ asthey are spaced further apart from the electric motors.

However, during operation, the robotic lawnmower may also operate inclose vicinity of the border cable 250. At such close distances, thepredominant interference is from neighbouring systems. Traditionally,the robotic lawnmower is then configured to synchronize on the rearsensor(s) 170″ as they would be further away from the neighbouringsystems.

The traditional method thus detected an amplitude of the receivedsignal, and adapted which sensor(s) to synchronize on accordingly.However, the inventors have realized that this manner of selecting whichsensor to synchronize on does not provide the optimum synchronizationwhen the work area comprises overlapping or stacked (adjacent) workareas. The inventors have also realized that this synchronization schemedid not perform optimally when following a boundary cable.

The inventors have therefore devised an improved manner of selectingwhich sensor(s) to synchronize on. In this improved manner, thecontroller 110 of the robotic lawnmower 100 is configured to compare thesignal (possibly obtained through autocorrelation) of one sensor 170 toa received reference sensor level. The received reference sensor levelmay be the total signal level, or the received reference sensor levelmay also or alternatively be the average signal level of all sensors170.

A signal qualifier may thus be used to determine which sensor to use.

The signal qualifier may be related to a quality of the received signaland/or a correlation value of the received signal when correlated to areference signal.

The sensor which has the highest signal qualifier, will be the sensorused for synchronization. This ensures that the best synchronisation isachieved irrespective of the current situation or location.

It also enables the robotic lawnmower to synchronize to which ever cableor field that happens to provide the best signal level, be it the guidecable, the boundary cable, the F-field or the N-field.

This provides for a more robust synchronization that can adapt tochanging environments. It also provides a synchronisation that is morebeneficial for use with overlapping or adjacent work areas.

The controller is thus configured to receive a signal through a firstsensor 170′ and to determine a first signal level for the first sensor170′.

In one embodiment the signal qualifier is determined based on theaverage of received signal qualifiers from signals received through allsensors 170.

In one embodiment the signal qualifier is determined based on the totalof received signal qualifiers from signals received through all sensors170.

The controller is also configured to receive a signal through a secondsensor 170″ and to determine a second signal qualifier for the secondsensor 170″.

The sensor 170 providing the highest signal qualifier is then selectedfor synchronisation.

In one embodiment the signal level is determined through autocorrelating with a library signal. In one such embodiment, the qualityof the received signal is determined using the formula below:

${{signal}\mspace{14mu} {quality}\mspace{14mu} {level}} = \frac{\frac{\sum\limits_{1}^{N}{c_{i}*x_{i}}}{N}}{\sqrt{\frac{\sum\limits_{1}^{N}x_{i}^{2}}{N}}}$

where

N=Frame length (for example 2047)

c_(i)=value at time i of the library signal

x_(i)=value at time i of the received or picked up signal

The flowchart of FIG. 7 shows a method according to the teachingsherein, where an robotic lawnmower receives 710 a signal through a firstsensor 170′ and determines 720 a first signal qualifier for the firstsensor 170′.

In one embodiment the signal quality level is determined based on theaverage of received signal quality levels from signals received throughall sensors 170.

In one embodiment the signal quality level is determined based on thetotal of received signal quality levels from signals received throughall sensors 170.

The robotic lawnmower also receives 730 a signal through a second sensor170″ and determines 740 a second signal quality level (or quality) forthe second sensor 170″.

The robotic lawnmower then compares the first and second signal qualitylevels 750 and the sensor 170 providing the highest signal quality levelis then selected for synchronisation 760.

As has been discussed in the background section, the work area and theboundary cable of a robotic lawnmower system may be modelled as anRL-circuit, where the R and L components (resistance R and inductance L)depend on the physical characteristics of the work area, including thecharacteristics of the ground and the size of the area. As such, and asthe inventors have realized, this makes it difficult when transmittingCDMA signals, as the decoding on the receiver side need to be highlyadvanced for correct decoding of the received signals, as the receivedsignal will differ depending on which work area it is used in. As theinventors have realized, an RL-circuit will act as a low-pass bandfilter when transmitting signals with varying or several frequencycomponents. This will, as is also mentioned in the background section,attenuate the higher frequency components, which may lead to thatsignals having many frequency components, such as CDMA signals, may beincorrectly received or decoded.

Assuming that all frequencies are transmitted at the same amplitude, theRL-circuit (composed by the work area) will filter the frequenciesdifferently, and the received signal will be difficult to decodecorrectly. The amount of filtration will depend on the RL-componentsmodelling the actual work area.

Instead of simply and straightforwardly increasing the voltage level forthe higher frequency components, which would also enable the CDMAsignals to be correctly received, the inventors have proposed a moreclever and ingenious manner of providing the boundary signals. Theinventors firstly propose to use a current generator instead of avoltage generator traditionally used. Secondly, the inventors propose toprefilter the transmitted signal corresponding to a model of the workarea. The prefiltering basically means that the higher frequenciesshould be transmitted at lower amplitudes, as in FIG. 8 showing afrequency-amplitude diagram for a transmitted signal It.

This enables the CDMA signal to be correctly received by the receiverwithout increasing the overall field strength too much.

As the inventors have realized, this is made possible by the fact thatthe receiver side reacts to the derivate of the signal (i.e. to thechanges in the signal). As the higher frequency components change morerapidly, the sensors will also pick them up more easily. The receivedsignal will thus look as in the schematic view of FIG. 9.

Thus by modelling the work area on an RL-circuit and adapting a currentgenerator to transmit a signal accordingly, an improved reception of theCDMA signal is provided. And, this is achieved without increasing thepower at which the high frequency components/signals are transmitted.

By transmitting the signal in the manner shown in FIG. 8, the receptionbecomes substantially indifferent to the RL-modelling of the work area.

In one embodiment, the controller of the robotic lawnmower may beconfigured to determine a signal level for a low frequency component andto determine a signal level for a high frequency component and based onthis, the RL-model used by the current generator may be adaptedaccordingly, whereby the output for the current generator is adapted tofit the RL model as based on the measurements made by the roboticlawnmower. Even though only two signal levels are mentioned herein, theskilled person would realize that to provide an accurate model, severalsignal levels could be needed.

The measurements made by the robotic lawnmower may be communicatedwirelessly through an RF interface (if such is comprised in the roboticlawnmower), or through the charging plates upon docking in the chargingstation.

The signal to be transmitted through the boundary cable may thus bepre-filtered according to the actual RL-model of the work area, beforebeing transmitted by the current generator.

Although the description herein have been focussed on an RL-model forthe prefiltering representing the work area, it should be understoodthat the RL-model may also comprise other components, such ascapacitance.

In one example the resistance, that is the R component, of the filter is20 Ohm, and the inductance, that is the L component, of the filter is 20mH.

FIG. 10 shows a schematic flowchart for a general method according tothe teachings herein, where a signal generator is configured toprefilter a signal to be transmitted through a model filter 1010, andthen transmit the signal using a current generator 1020.

As the inventors have also realized, in order to improve the receptionof higher frequency components, the current generator may be arranged tooperate by transmitting the signal at a constant current level. Thiswill enable the high frequency components to be received at a highersignal level, and therefore be less susceptible to noise and otherinterference, which is preferable when using CDMA signalling. This isalso achieved by not transmitting at higher power levels compared tocontemporary systems, thereby reducing or at least not increasing theinterference of other, neighbouring systems and devices.

The realization of using a current generator to transmit a prefilteredsignal provides for an improved manner of transmitting CDMA signals thatreduces or at least not increases the interference of other,neighbouring systems and devices.

As the signal is transmitted through the boundary cable, there might bea leak of current through stray capacitances that occur between theboundary cable and the surrounding environment. This can lead to thatcurrents are flowing through the boundary cable affecting the quality ofthe signal being transmitted there through. The leaking current may alsocreate a negative magnetic field that may fool the robotic lawnmower tobelieve it is operating outside the boundary cable 250, when in fact itis still inside the work area 205. This may result in that the operationof the robotic lawnmower is hindered or cancelled. The inventors haverealized this problem and have, after insightful reasoning, come up witha simple solution that does not require any change or addition to theboundary cable, which would increase the cost of the robotic lawnmowersystem and also complicate the installation of the robotic lawnmowersystem. The inventors are proposing to adapt the synchronization done bythe robotic lawnmower so that as it detects that it is sufficientlyclose to a guide cable G/260, sufficiently meaning being able to receivea signal with good signal quality (or at least better than the qualityat which the control signal being transmitted through the boundary cableis received) and far from the boundary cable (i.e. at a distance wherethe signal amplitude level is below a threshold value or lower than thatof the guide cable and/or where the signal quality level is below athreshold value or lower than that of the guide cable), the roboticlawnmower is configured to start synchronizing on the guide signalinstead of the boundary signal. This enables the robotic lawnmower tosafeguard against being fooled by the stray capacitances and leakingcurrents to believe it is outside the area when in fact it is not.

In one embodiment, the robotic lawnmower is configured to only listen tothe low frequencies of the control signal transmitted through theboundary cable. As mostly the higher frequencies are affected by theseleaking currents, only listening to the lower frequencies reduces therisk of being fooled. The robotic lawnmower will thus still be able todetermine that the boundary cable is still present, even whensynchronizing on the guide signal. To listen to both the guide signaland to the lower frequencies of the boundary signal also allows fordetermining the correct position of the robotic lawnmower, even insituations such as when the guide cable is actually outside the workarea. It also allows the robotic lawnmower to determine that the roboticlawnmower is closer to the boundary cable, where the effects of thestray capacitances are smaller and then switch over or back tosynchronize on the boundary signal again. Examples of such low frequencyranges are 0.5 kHz to 2 kHz in a signal having a frequency range of forexample 0.5 kHz to 7 kHz, Other examples is that the low frequenciescorrespond to the lower 15% of the frequencies in the signal, the lower20% of the frequencies in the signal, the lower 25% of the frequenciesin the signal, the lower 30% of the frequencies in the signal, the lower35% of the frequencies in the signal or the lower 50% of the frequenciesin the signal.

FIG. 11 shows a flowchart for a general method according to theteachings herein where a robotic lawnmower is configured for receiving1110 the control signal and determining 1120 a signal level for thecontrol signal; and if the signal level for the control signal is below1130 a threshold value, the robotic lawnmower receives said guide signaland synchronizes 1140 on said guide signal.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope of the invention, as defined by the appendedpatent claims.

1. A robotic lawnmower system, comprising a robotic lawnmower and acharging station, said charging station comprising a signal generatorbeing configured to transmit a boundary signal utilizing CDMA codinghaving a frame length corresponding to a full frame length through aboundary wire, wherein said signal generator is configured to transmitsaid boundary signal in subframes, and wherein said robotic lawnmower isconfigured to receive said boundary signal by detecting magnetic fieldsgenerated by the boundary signal, and wherein the robotic lawnmower isconfigured to determine a set of conditions; and to determine if saidset of conditions correspond to a first set of conditions, and if solisten to the subframes, and to determine if said set of conditionscorrespond to a second set of conditions, and if so listen to the fullframes.
 2. The robotic lawnmower system of claim 1, wherein the roboticlawnmower is further configured to determine that the set of conditionscorrespond to a third set of conditions, and in response thereto listento the subframes and the full frame,
 3. The robotic lawnmower system ofclaim 1, wherein the first set of conditions include a received highsignal quality level.
 4. The robotic lawnmower system of claim 1,wherein the first set of conditions include a received high signalstrength level.
 5. The robotic lawnmower system of claim 1, wherein thesecond set of conditions include a received low signal quality level. 6.The robotic lawnmower system of claim 1, wherein the second set ofconditions include a received low signal strength level.
 7. The roboticlawnmower system of claim 2, wherein the third set of conditions includea received high signal strength level and low signal quality level. 8.The robotic lawnmower system of claim 1, wherein the signal generator isfurther configured to transmit the subframes in a nonorderly fashion. 9.A method for use in a robotic lawnmower system, comprising a roboticlawnmower and a charging station, said charging station comprising asignal generator being configured to transmit a boundary signalutilizing CDMA coding having a frame length corresponding to a fullframe length through a boundary wire, wherein said signal generator isconfigured to transmit said boundary signal in subframes, and whereinsaid robotic lawnmower is configured to receive said boundary signal bydetecting magnetic fields generated by the boundary signal, and whereinthe method comprises determining a set of conditions; and determining ifsaid set of conditions correspond to a first set of conditions, and ifso listen to the subframes, and determining if said set of conditionscorrespond to a second set of conditions, and if so listen to the fullframes.